The present invention relates to an organic electroluminescence element used in light emitting element or the like applied in various display devices, light source or backlight of display device, or optical communication appliances, its manufacturing method, and displaying device using an organic electroluminescence element.
An electroluminescence element is a light emitting device making use of electric field light emission of solid fluorescent substance or phenomenon called electroluminescence, and at the present an inorganic electroluminescence element using an inorganic material as luminous body is practically used, and it is applied in the backlight of liquid crystal display, flat display, etc. However, the inorganic electroluminescence element requires alternating current and a high voltage of over 100 V in order to illuminate the element, and blue color light emission is difficult, and it is hence difficult to realize full color of R, G, B, that is, three primaries of red, green and blue.
On the other hand, the research about electroluminescence element using organic material has been long attempted, but the efficiency is very poor, and only low luminance is obtained, and it is far from practical study. However, the structure proposed by C. W. Tang et al. of Kodak in 1987, that is, the organic electroluminescence element having a laminate structure of function separate type dividing the organic substance into two layers of hole transfer layer and luminous layer 5 realized a high luminance light emission of over 1000 cd/m.sup.-2 in spite of low voltage of 10 V or less (C. S. Tang and S. A. Vanslyke: Appl. Phys. Lett, 51 (1987) 913), where cd is an abbreviation of candela. Then the attention has been turned to the organic electroluminescence, and the laminate type organic electroluminescence elements having similar structure have been intensively studied recently.
The composition of a conventional organic thin film electroluminescence element is briefly described in FIG. 14.
In FIG. 14, an electroluminescence element is composed of a substrate 61, an anode 62, an organic thin film layer 63, a hole transfer layer 64, a luminous layer 65, and a cathode 66.
As shown in FIG. 14, a conventional organic electroluminescence element comprises a transparent or translucent substrate 61 of glass or the like, an anode 62 made of a transparent conductive film such as ITO formed on the substrate 61 by sputtering method, vapor deposition method or the like, a hole transfer layer 64 made of TPD formed on the anode 62 by vapor deposition method or the like, that is, N,N'-diphenyl-N,N'-bis(3-methyl phenyl)-1,140 -diphenyl-4,4'-diamine (hereinafter called TPD), a luminous layer 65 formed on the hole transfer layer 64 by vapor deposition method or the like, being composed of 8-hydroxy quinoline aluminum (hereinafter called Alq3) or the like, and a cathode 66 made of metal film or the like formed on the luminous layer 65 by vapor deposition method or the like.
In the organic electroluminescence element shown in FIG. 14, an organic thin film layer 63 is composed of the hole transfer layer 64 and luminous layer 65.
Using the anode 62 of the organic electroluminescence element having such constitution as the positive electrode and the cathode 66 as the negative electrode, when a direct-current voltage or direct-current current is applied, holes are injected into the luminous layer 65 from the anode 62 through the hole transfer layer 64, and electrons are injected into the luminous layer 65 from the cathode 66. In the luminous layer 65, re-bonding of holes and electrons takes place, and hence the produced exciters are moved from the excited state to the basal state, and a luminous phenomenon occurs at this time. By varying the laminate structure for composing the organic thin film layer 63 or the material used in the luminous layer 65, the luminous wavelength can be changed. To enhance the luminous characteristic of such organic electroluminescence element, so far, (1) improvement of composition of the organic thin film layer 63 such as luminous layer 65 and hole transfer layer 64 or organic material used for them, or (2) improvement of material used for the anode 62 or cathode 66 had been studied.
For example, as for (2), in order to lower the barrier of the cathode 66 and luminous layer 65 for the ease of injection of electrons into the luminous layer 65, the Mg--Ag alloy was proposed in U.S. Pat. No. 4,885,211, or the material of small work function and high electric conductivity such as Al--Li alloy was disclosed in Japanese Laid-open Patent 5-121172, and these materials are widely employed at the present.
However, such alloy materials are high in activity and unstable chemically, they are corroded or oxidized by reaction with moisture or oxygen in the air. Such corrosion or oxidation of the cathode 66 causes to grow extremely the non-luminous area called dark sports existing in the luminous layer 65, which is a cause of transitional deterioration of characteristic in the organic electroluminescence element.
Not limited to the cathode 66, yet, the organic material used in organic thin film layer 63 such as luminous layer 65 and hole transfer layer 64 also changes in structure generally by reaction with moisture or oxygen, which may also give rise to growth of dark spots.
Therefore, to enhance the durability and reliability of organic electroluminescence element, it is necessary to seal the entire organic electroluminescence element in order to prevent reaction of the material used in the cathode 66 or organic thin film layer 63 with moisture or oxygen.
For sealing of the organic electroluminescence element, mainly two methods have been studied so far. One is to form a protective film on the outer surface of the organic electroluminescence element by using vacuum film forming technology such as vapor deposition method, and other is to adhere a shield material 8 made of glass cap or the like to the organic electroluminescence element.
The method of sealing the organic electroluminescence element by forming a protective film is disclosed, for example, in Japanese Laid-open Patent 6-96858, which relates to a method of forming GeO, SiO, AlF3, or the like on the outer surface of the organic electroluminescence element by ion plating method. In Japanese Laid-open Patent 7-211455, a method of forming a protective film composed of a moisture absorbing material with moisture absorption of 1% or more and a moisture-proof material with moisture absorption of 0.1% or less is disclosed.
As the method of sealing the organic electroluminescence element by adhering a shield material, as already employed in the inorganic electroluminescence element, a method of installing a glass plate outside of the back electrode and filling the space between the back electrode and glass plate with silicone oil is known. Besides, Japanese Laid-open Patent 5-089959 discloses a method of shielding by electric insulating glass or electric insulating airtight fluid after forming a protective film composed of insulating inorganic compound. In the case of shielding by electric insulating airtight fluid, a method of filling a glass container or the like with inert gas or silicone oil, and sealing the opening with epoxy resin is presented.
The present inventors investigated growth of dark spots from various viewpoints, and discovered that growth of dark spots is promoted even by trace of moisture existing in the vacuum of, for example, 10.sup.-4 Torr.
Causes of occurrence of dark spots originate mainly from contamination on the anode such as ITO film and dust deposits on the substrate 61. In the case of ITO film, its surface contamination can be nearly eliminated by devising a proper cleaning method, but it is difficult to completely eliminate dust deposits on the substrate 61. For example, if the organic electroluminescence element is manufactured on a clean room, even in a clean room of class 100, one dust particle of about 3 microns is present in 10 liters. Also multiple dust particles are present in the vapor deposition apparatus used in the manufacturing process, or dust may deposit on the substrate 61 when forming a film. Therefore, if working in a clean room of an advanced degree of cleanliness, dust is present on the substrate 61 at a considerably high rate, and it is extremely difficult to prevent completely generation of dark spots.
In addition, since dark spots are caused by dust particles of several microns existing on the substrate 61, in the conventional organic electroluminescence element composed of organic thin film layer 63 of about 0.1 micron, cathode 66 of about 0.2 micron, and protective film of about 0.5 micron formed thereon, the total film thickness is about 1 micron, and dust cannot be concealed completely. Therefore, if the protective film itself is completely impermeable not to pass oxygen or moisture, unless the dust is completely covered by the protective film, oxygen or moisture may invade into the organic thin film 63 or cathode 66 from the surrounding of dust, so that dark spots may be grown.
Hence, to eliminate growth of dark spots completely, it is necessary to shut off nearly completely invasion of moisture or oxygen into the material used in the cathode 66 or organic thin film layer 63.
Incidentally, as for the organic material used in the organic electroluminescence element, the upper limit of the heating temperature allowed in the manufacturing process is about 100.degree. C. Therefore, this temperature cannot be exceeded if forming protective film by vapor deposition method or the like , but as for the oxide film such as GeO, SiO, and SiO.sub.2 used as protective film, it is hard to form a sufficiently dense film at low temperature of about 100.degree. C. generally, and in such film forming condition, multiple defects and pin holes are present in the film, and moisture and oxygen cannot be shut off completely. If attempted to solve these problems by increasing the film thickness, the internal stress of the protective film increases as the film thickness increases, the damages are given to the cathode 66 or organic thin film layer 63, which may possibly lead to lowering of light emitting luminance or short-circuit of the organic electroluminescence element.
Besides, sealing by shield material hitherto attempted has not completely suppressed growth of dark spots. The epoxy resin used in adhesion with the electric insulating glass substrate mentioned in the above Japanese Laid-open Patent 5-089959 generally has a steam permeability if about 3 to 5 (g/m.sup.2.24 h/mm), or polyimide resin, about 2 (g/m.sup.2.24 h/mm), and hence invasion of water from the adhesion portion cannot be suppressed completely.
Thus, by the protective film or adhesion by glass cap used hitherto in the organic electroluminescence element, it was impossible to completely suppress growth of dark sports.